Pressure vessel



United States Patent Oflice 3,393,731 PRESSURE VESSEL Irwin R. Friedmanand George P. Staats, La Crosse, Wis., assignors to The Trane Company,La Crosse, Wis., a corporation of Wisconsin Filed May 12, 1967, Ser. No.638,063 8 Claims. (Cl. 165-158) ABSTRACT OF THE DISCLOSURE A pressurevessel assembly having at least two opposed end walls of curved crosssection wherein the end walls Background of the invention In designingvessels to contain relatively high fluid pressures where size and weightare important considerations, various means have been employed to reducethe thickness of the materials of which the vessel is constructed. Thesuperior pressure containing characteristics of curved wall sections areknown. Even where curved wall sections are employed, however, thereexists a problem of minimizing the bending stresses on the curved wallsin order that walls of minimum thickness may be employed. When a coverplate is bolted over an access opening in the end of a curved pressurevessel, bending moments introduced in the cover plate and in the vesselportion to which it is attached normally necessitate thick cover platesand reinforcing thicknesses of metal at the joints between the coverplate and the vessel. Thick, heavy cover plates are diflicult to remove,and often require special lifting gear.

Summary of the invention Having in mind the foregoing problemsassociated with the design of pressure vessels, we have designed apressure vessel utilizing the pressure containing strength of curvedsections in an essentially rectangular shaped vessel in such a way as tominimize material requirements and weight. This is accomplished byplacing as many members in pure tension as possible and by eliminatingor reducing bending moments by balancing tension forces. According toour invention, bending forces on curved wall sections are minimized byutilizing tension carrying tie bar members to secure one edge of thecurved wall sections to a back plate. The curved walls and tie barmembers are interconnected by means of flange bars to which both thecurved walls and the tie bar members are welded. The flange bars alsoserve to support a removable cover plate which is bolted to the flangebars. The fluid pressure acting outwardly on the flange bars through thecover plate bolts is balanced by the counteracting tension forces in thetie bars and in the curved end sections so that there is substantiallyno net bending force acting on the curved wall sections. This permitsthe use ofv curved wall sections of minimum thickness.

Another advantageous feature of our improved pressure vessel lies in theselection of the angle of contact and contact location of the curvedwall sections on the flange bars so that torsional moments acting on theflange bars are minimized. Also, the required thickness 3,393,731Patented July 23, 1968 for the removable cover plate is substantiallyreduced and the overall strength of the pressure vessel assembly isimproved by utilizing a baflle plate which separates the vessel into twoseparate fluid chambers to secure the cover plate to a back plate.

These design features are particularly advantageous when applied to arectangular chamber having a small depth compared to its length. We haveincorporated these features in a pressure vessel of such a shape, whichis particularly adapted for use as a header on a shell and tube heatexchanger.

These and other features and advantages of our invention will becomereadily apparent as the following description is read in conjunctionwith the attached drawmgs.

Brief description of the drawings FIGURE 1 is a front elevation view ofthe improved pressure vessel of this invention.

FIGURE 2 is a vertical cross-section view taken along line 2-2 of FIGURE1.

FIGURE 3 is a perspective view of the pressure vessel with the frontcover plate removed.

FIGURE 4 is a blown-up section view showing the various force vectorsacting on one of the flange bars which joins the front cover plate, oneset of the tie bars and one of the curved end walls together.

Description of the preferred embodiment Our improved pressure vessel isshown in FIGURES 1 through 3 as a header 2 closing one end of a heatexchanger comprised of a cylindrical shell 1 and a plurality of tubes 5extending lengthwise thereof. Tube sheet 4 is welded or otherwisesecured to one end of shell 1, and is provided with a plurality of holesthrough which tubes 5 pass. Tube sheet 4 serves as the substantiallyplanar back wall of rectangular-shaped header assembly 2. In fabricatingheader assembly 2, longitudinally extending flange bars 20 and 22 arefirst welded lengthwise between the front edges 16 and 18 of flat sidewalls 6 and 8 respectively. Flange bar 34 is also welded between thefront edges of side plates 6 and 8 parallel to and between flange bars20 and 22. Baflle plate 32 is then welded to the inside edge of flangebar 34 and to opposed side walls 6 and 8 along its side edges. Baffleplate 32 extends generally perpendicular to side plates 6 and 8, andforms therewith a generally H-shaped subassembly. This entiresubassembly is then secured to one end of the heat exchanger by weldingthe inner edges of side plates 6 and 8 and baffle plate 32 to tube sheet4.

Next, a plurality of spaced apart tie bars 28 are welded across the topof heat exchanger header assembly 2 between tube sheet 4 and flange bar20. A second set of spaced apart tie bars 30 are secured between tubesheet 4 and bottom flange bar 22. Each of the side plates 6 and 8 arecurved along their top edges 9 and 9a and bottom edges 10 and 10a. Twoopposed end walls 12 and 14 of curved cross-section extend across thetop and bottom of header assembly 2 between side walls 6 and 8. Endwalls 12 and 14 are curved to conform to curved top and bottom edges 9,10 and 9a and 10a of side plates 6 and 8 to which they are welded.Curved end walls 12 and 14 are also welded along their back edges totube sheet 4.

Spaced apart flange bars 20 and 22 are coplanar with front edges 16 and18 of side plates 6 and 8, and define therewith a front access opening.This access opening is normally closed by front cover plate 24 which isremovably fastened to flange bars 20 and 22 to front edges 16 and 18 ofside plates 6 and 8 by bolts 26. Cover plate 2-4 is also fastened tointermediate flange bar 34 by bolts 36.

Curved end walls 12 and 14 are preferably cylindrically shaped, and havea small radius of curvature so that the lightweight materials of whichthey are made can contain relatively high pressures. Three-sixteenthinch to 4 inch plates were used for curved end walls 12 and 14 in placeof the l to 1 /2" plates normally required in conventional designshaving flat end walls. Flange bars 20 and 22 serve as structural membersinterconnecting the front edges of curved end walls 12 and 14 to theouter ends of tie bar members 28 and 30. The major portion of theoutward thrust generated by the pressure of the fluid within headerassembly 2 one end walls '12 and 14 is thus carried by tie bars 28 and30. In order to effectively eliminate any significant bending moment oncurved end walls 12 and 14, the tension forces acting on the front edgesof end walls 12 and 14 are balanced. The manner in which this isaccomplished can best be understood by referring to the force vectorsillustrated in FIGURE 4. The reaction tension force F acting through endwall 12 has been resolved into its vertical and horizontal components Fand F The fluid pressure acting outwardly on cover plate 24 throughbolts 26 is represented by force vector P and the reaction tension forceacting through tie bars 28 is shown by vector F Vertical component P ofF is balanced by a shear force P on bolts 26 transmitted to cover 24through the bolts. The angle on at which curved end wall 12 is connectedto flange bar 20 is so selected that the horizontal force component P ofwall 12 combined with tension force F in bars 28 equals the opposingforce F through bolts 26. This relationship of tension forces may beexpressed algebraically by the equations:

With a radius of curvature of 127 for end walls 12 and 14, angle a atwhich they contact flange bars 20 and 22 will be approximately 30. Thisbalancing of tension forces effectively precludes any net bending stressacting on curved end -walls 12 and 14. This permits these walls to bedesigned with a view towards only the membrane stress resulting from thefluid pressure within header assembly 2, and thus to be relatively thin.

The point of contact of curved end walls 12 and 14 on flange bars 20 and22 respectively is selected so as to minimize the net torsional stressacting on these flange bars. The net torsional stress acting on flangebar 20', for example, can be expressed algebraically by summing themoments about a particular point on bar 20. With end wall 12 contactingthe top of flange bar a distance r from the front face of bar 20, thesum of the moments about point 0 in the vertical center of the frontface of bar 20 will be:

Sum of wherein forces F and F are acting at distances 1' and r frompoint 0, and Me is the torsional moment of the front cover acting on theflange bar 20 as a result of the fluid pressure Within header 2. Thedistance 1' and the angle or for cylindrical end walls 12 and 14 for aparticuular size header assembly are selected so that the net torsionalstress on flange bars 20 and 22 will be a minimum value. Inmanufacturing and assembly 2, a tolerance is established for distance 1'which will produce a net torsional load on flange bars 20 and 22 whichdoes not exceed the minimum torsional strength for which they aredesigned. By minimizing the torsional stress acting on flange bars 20and 22, the dimensions for these members may be determined primarily bybolting and gasketing requirements rather than on the basis of strengthconsiderations. This design feature is particularly useful on long,narrow pressure vessels having relatively long flange bars 20 and 22 onwhich the cumulative torsional stress would be very great. Balancingtorsional moments by means of the aforesaid design arrangement of curvedend walls 12 and 14 with respect to flange bars 20 and 22 permitsrelatively small flange bars to be used in place of the thick, heavybars which would normally be required on header assemblies of such ashape. For example, with a design pressure of p.s.i. and a rectangularshaped header approximately 56 inches long and 17% inches deep, flangebars 1 /2 inches thick may be used. If the angle of contact a for endwalls "12 and 14 of a header of such size is 30, then dimension r willbe about 1% inches.

In the example given above showing the torsional moments acting onflange bar 20, a minimum net torsional stress was achieved by having thethrust load carried by tie bars 28 act along bottom face 21 of flangebar 20. For this purpose, and for welding convenience, tie bars 28 areshown lying against face 21 of tie bar 20. The same result could beobtained by abutting tie bars 28 against back face 23 of bars 20 in thesame plane as bottom face 21. However, this would require cutting tiebars 28 to a particular length so that they fit exactly in the spacebetween tube sheet 4 and back face 23 of tie bar 20. It is possible thatin order to obtain the proper balancing of torsional moments for varioussize pressure vessels, tie bars 28 would have to be welded farther upalong back face 23 of flange bar 20 intermediate its top and bottomfaces. Of course, the same considerations apply to the determination ofthe proper point for welding tie bars 30 to flange bar 22.

When our improved pressure vessel is employed as a header assembly asillustrated, bafile plate 32 serves to divide the header interior spaceinto inlet and outlet fluid chambers. Flanged fluid inlet and outletfittings 38 and 40 are provided in flat side wall 8 above and belowdividing baffle 32. Side walls 6 and 8 could also be cylindricallyshaped, as are end walls 12 and 14. However, this would make it moredifficult to secure fittings 38 and 40 to one of the side walls. Thisarrangement wherein the fluid connections and cover plate are indifferent walls is the normal marine type of water box design whichpermits access to tubes 5 for cleaning and replacement withoutdisturbing the pipes connected to fittings 38 and 40. Heat exchangefluid is directed into header 2 through inlet fitting 40, down throughtubes 5 in One direction inside of shell 1, back through tubes 5 in theopposite direction into the upper part of header 2 above chamberseparation bafile 32 and out through fitting 38. A second heat exchangefluid is directed through shell 1 in thermal contact with tubes 5 in thewell known manner. Since baffle plate 32 is welded to flange bar 34which in turn is bolted to front cover plate 24, plate 32 also acts as athrust carrying member lending support to cover plate 24. By thusreducing the strength requirements for cover plate 24, its thickness maybe reduced to half that normally required. Cover plate 24 will thus besufliciently lightweight that it can be removed without the aid ofspecial overhead hoists. Also, the tie bar effect of plate 32 furtherreduces the torsional moment Me of cover 24 on flange bars 20 and 22 aswell as the thrust load on bars 28 and 30.

A pressure vessel fabricated as described above has the particularadvantage of high strength coupled with light weight. By placing as manymembers as possible in pure tension and minimizing bending and twistingstresses by balancing tension forces, material requirements are greatlyreduced. The H-frame assembly formed by side plates 6 and 8 and bafileplate 32 produces an extremely rigid structure. The mutual support whichtube sheet 4 and front cover 24 provide for each other through thisinterconnecting I-I-frame assembly permits the thickness of the tubesheet and the front plate to be drastically reduced.

Although we have illustrated and described our pressure vessel as aheader for use on a heat exchanger, the design principles pointed outabove could be applied to any vessel and could be adapted to asymmetrical design bolted together in the middle.

Various modifications will occur to those skilled in the art which wecontemplate will be within the spirit and scope of our invention asdefined by the following claims.

We claim:

1. In combination with a heat exchanger comprising a shell and aplurality of tubes extending therein, a generally rectangular headerassembly comprising:

a tube sheet secured to and closing one end of said shell defining theback wall of the header assembly;

a substantially planar front wall;

two opposed side walls extending between said front and back walls, eachof said side walls being curved along the opposed edges thereof whichextend between said front and back walls;

two opposed end walls of curvilinear cross section, said end walls beingcurved to conform to said curved edges of said side walls and extendinglengthwise of said header assembly between said opposed side walls inmating, abutting relationship therewith;

a first set of spaced apart tie bar members secured at one end to saidback wall;

first means securing the other end of each of said first set of tie barmembers to the front edge of one of said curved end walls;

a second set of spaced apart tie bar members secured at one end to saidback wall;

and second means securing the other end of each of said second set oftie bar members to the front edge of the other one of said curved endwalls.

2. Apparatus as defined in claim 1 wherein:

said first and second means securing the other end of each of said tiebar members of said first and second sets to the front edge of each ofsaid curved end walls comprise first and second spaced apart flange barsextending lengthwise of said header assembly between said side walls,said front edge of said one of said curved end walls and each of saidfirst set of tie bar members being welded to different faces of saidfirst flange bar, and said front edge of said other one of said curveend walls and each of said second set of tie bar members being welded todifferent faces of said second flange bar;

and wherein said planar front wall is secured to said first and secondflange bars.

3. Apparatus as defined in claim 2 wherein:

each of said first set of tie bar members lies against a face of saidfirst flange bar which is opposite the face to which said front edge ofsaid one of said curved end walls is welded;

and wherein each of said second set of tie bar members lies against aface of said second flange bar which is opposite the face to which saidfront edge of said other one of said curved end walls is welded.

4. Apparatus as defined in claim 2 wherein:

the contact angle and point of contact of said front edges of each ofsaid curved end walls with respect to the faces of said first and secondflange bars to which said front edges are welded are such that the nettorsional stress on each of said flange bars approximates zero.

5. Apparatus as defined in claim 2 and further includa third flange barpositioned between and parallel to said first and second flange bars,said planar front wall being secured to said third flange bar;

a bafile plate extending generally longitudinally of said headerassembly, said baflle plate being secured along its edges to said thirdflange bar, said back wall and said side walls;

and fluid inlet and outlet openings in said back wall and in one of saidside walls, said inlet opening being located on the opposite side ofsaid baflle plate from said outlet opening, whereby said baffle plateserves both as a separation member dividing said header assembly intofluid inlet and outlet chambers and as a thrust carrying member lendingsupport to'said front wall.

6. Apparatus as defined in claim 5 wherein:

said planar front wall is in the form of a lightweight cover plateremovably secured to said first, second and third flange bars.

7. Apparatus as defined in claim 1 wherein:

said first and second means securing said other ends of said first andsecond sets of tie bar member to said front edges of said curved endwalls comprises first and second flange bars extending lengthwise ofsaid header assembly between said side walls, said first flange barbeing welded to said front edge of said one of said curved end walls andto said other end of each of said first set of tie bar members, and saidsecond flange bar being welded to said front edge of said other one ofsaid curved end walls and to said other end of each of said second setof tie bar members;

and wherein said flange bars are coplanar with the front extremities ofsaid side walls and define therewith an access opening;

and wherein said planar front wall is in the form of a cover plateremovably secured over said access opening to said flange bars.

8. In combination with a heat exchanger comprising a shell and aplurality of tubes extending therethrough, a header assembly comprising:

a tube sheet secured to and closing one end of said shell;

planar side wall means and curvilinear end wall means secured to saidtube sheet and extending forwardly therefrom, the forward extremities ofsaid side wall means and said end wall means terminating in flange meanswhich define a front access opening;

a cover plate removably mounted on said flange means over said accessopening;

means dividing said header assembly into two separate fluid chambers,said means comprising a baflle plate secured at its back edge to saidtube sheet;

means securing the forward edge of said baflle plate to said coverplate, whereby said baffle plate serves as a thrust carrying membersupporting said cover plate;

and tie bar members secured between said tube sheet and said flangemeans at the forward extremity of said end wall means.

FOREIGN PATENTS 12/1952 Great Britain. 12/1959 Great Britain.

ROBERT A. OLEARY, Primary Examiner.

A. W. DAVIS, JR., Assistant Examiner.

